Block by internal Mg²⁺ causes voltage-dependent inactivation of Kv1.5

Abstract

Internal Mg²⁺ blocks many potassium channels including Kv1.5. Here, we show that internal Mg²⁺ block of Kv1.5 induces voltage-dependent current decay at strongly depolarised potentials that contains a component due to acceleration of C-type inactivation after pore block. The voltage-dependent current decay was fitted to a bi-exponential function (τ_fast and τ_slow). Without Mg²⁺, τ_fast and τ_slow were voltage-independent, but with 10 mM Mg²⁺, τ_fast decreased from 156 ms at +40 mV to 5 ms at +140 mV and τ_slow decreased from 2.3 s to 206 ms. With Mg²⁺, tail currents after short pulses that allowed only the fast phase of decay showed a rising phase that reflected voltage-dependent unbinding. This suggested that the fast phase of voltage-dependent current decay was due to Mg²⁺ pore block. In contrast, tail currents after longer pulses that allowed the slow phase of decay were reduced to almost zero suggesting that the slow phase was due to channel inactivation. Consistent with this, the mutation R487V (equivalent to T449V in Shaker) or increasing external K⁺, both of which reduce C-type inactivation, prevented the slow phase of decay. These results are consistent with voltage-dependent open-channel block of Kv1.5 by internal Mg²⁺ that subsequently induces C-type inactivation by restricting K⁺ filling of the selectivity filter from the internal solution.